I believe in 'gender dysphoria' as a mental illness, but I think the number of people truly affected is tiny, and most of the people claiming to be affected today are probably not truly dysphoric.
This was written some years ago before it became transphobic to think that trans people were trans because of their gender dysphoria:
According to the DSM-5, among individuals who are assigned male at birth, approximately 0.005 percent to 0.014 percent are later diagnosed with gender dysphoria. Among individuals who are assigned female at birth, approximately 0.002 percent to 0.003 percent are later diagnosed with gender dysphoria. Because these estimates are based on the number of people who seek formal treatment—including hormone therapy and/or surgical reassignment—these rates are likely an underestimate of actual prevalence.
www.psychologytoday.com/gb/conditions/gender-dysphoria
If one assumed the numbers are still relevant today, one could ask that if less than 5% of trans people have gender dyshoria, what condition do the remaining 95+% have?